1. Field of the Invention
The present invention relates to an apparatus for forming an image through the use of an electrophotographic process. More particularly, the present invention is concerned with a developing device wherein a toner carrier is brought into pressure contact with a latent image carrier to develop an image.
2. Description of the Related Art
As disclosed in U.S. Pat. No. 3,731,146, a conventional developing device known in the art, in which a toner carrier is brought into pressure contact with a latent image carrier to develop an image, comprises a foam member as a backing material (a base material) and a soft electroconductive layer separately provided on the surface of the foam member to form a toner carrier. In this device, a toner is transported to the toner carrier and brought into pressure contact with a latent image carrier to develop an image.
In the above-described conventional toner carrier, however, in order to form an even electroconductive layer on the surface of a foam member as a base material, it is at least necessary to form an electroconductive layer so that the layer thickness is greater than the size of a cell of the foam member. However, it is not always easy to retain the flexibility of the foam member up to the surface of the toner carrier and exhibit the flexibility in that place. For this reason, a fluctuation in the environment or the like gives rise to a variation in the development pressure and developing nip width (contact width), so that there occur problems of a fluctuation in the density and a lowering in the resolution. Further, in the above-described toner carrier having a structure comprising two separate layers, that is, a base material and a surface layer, it is at least necessary to provide, in the manufacture thereof, a series of steps, i.e., at least a step of forming a foam member, a step of machining the foam member and a step of forming an electroconductive layer. Further, since the foam member is flexible, particularly the step of machining the foam member becomes complicated and inevitably requires a lot of time, which gives rise to an increase in the production cost of the toner carrier and in its turn an increase in the cost of the developing device.